Field of the Invention
The present invention relates to a variable directional microphone, the directionality of which can be switched from unidirectional to narrow directional and vice versa depending on an ambient noise level.
Description of the Related Art
In case of sound recording in an open-air environment, when an ambient noise level surrounding a targeted sound source is low, a unidirectional (such as a cardioid or a sub-cardioid directional) microphone is preferably used. In this case, high quality sound recording is possible while suppressing to record undesirable ambient sounds, even though there might be quality changes of sound depending on distance or the direction from the sound source.
In contrast, when the ambient noise level is high, use of a narrow directional microphone is preferred. In this case, the targeted sound wave can be surely recorded, even though there is considerable changes of sound quality depending on the distance or the direction from the sound source.
Thus, preferable directional characteristics of the microphones to be used are different depending on the surrounding condition. Therefore, a microphone the directionality of which can be switched from unidirectional to narrow directional and vice versa has been required; the microphone is operated as a unidirectional microphone when the ambient noise level is low, and operated as a narrow directional microphone when the ambient noise level is high. Particularly in news media, broadcasting with simple devices in comparison with the former ones becomes possible due to development of communication technologies. Therefore, live broadcasting can be more easily performed, which conventionally required large-scaled techniques. As for microphones in addition, it is convenient that the directionality of the microphone is easily switchable from unidirectional (a handy microphone case) to narrow directional (a shotgun microphone case) and vice versa, since consequently there is no need to prepare many kinds of microphones.
Japanese Patent Application Laid-Open (JP-A) No. 2000-50385 discloses a line microphone in which a unidirectional microphone unit is placed inside of a rear end side of an acoustic tube, and the microphone unit divides the acoustic tube into a front acoustic capacity room and a rear acoustic capacity room, and a rear sound-wave inlet-opening for a rear acoustic terminal of the microphone unit is additionally provided in the rear acoustic capacity room side.
In the above-said microphone, there is provided, on the area of the rear sound wave inlet-opening, a switching cover capable of varying the acoustic resistance of the rear sound wave inlet-opening, and switching the directionality to either hyper cardioid or cardioid, for example.
According to the microphone disclosed in the above referred patent application, the directionality can be switched between cardioid and hyper cardioid by changes of the acoustic resistance caused by variation of the area of the rear sound wave inlet-opening covered with the switching cover.
That is, by fully opening or fully closing the rear sound wave inlet-opening, predetermined directionality of cardioid or hyper-cardioid is instantaneously available, respectively.
To obtain intermediate directionality between the directionalities above, however, it is necessary to vary the area of the small rear sound wave inlet-opening by partially covering with the switching cover. Thus, it is difficult for users to vary the acoustic resistance of the rear acoustic terminal finely, adjustment of directionality is not easy.